Display console messages, if available

What does this MR do?

This MR introduces the ability to provide a console_message which will be displayed by gitlab-shell (gitlab-shell!287 (merged)) if present.

For CE, this is currently a no-op.

What are the relevant issue numbers?

• https://gitlab.com/gitlab-org/gitlab-ee/issues/9217
• EE version https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/10471
• gitlab-shell part gitlab-shell!287 (merged)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• [ ] Documentation created/updated via this MR
• [ ] Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• [ ] Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• [ ] Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer